Hydrogen will be one of the most important fuels in the future because it has the potential to be produced from renewable resources and can provide a clean and efficient fuel for fuel cells. Huber et al. have proposed a method to renewably produce hydrogen via the aqueous phase reforming (APR) of oxygenates. They have identified Pt-Ni catalysts as one of several bimetallic catalysts that display both high activity towards the APR reaction and high selectivity to the desired H2 product [1]. Identifying and designing a more active and selective catalyst for the reforming of oxygenates will make the production of H2 from renewable resources more efficient.
The reforming of oxygenates over single crystal bimetallic surfaces has also been studied by Skoplyak et al. in which the surface created by monolayer deposition of Ni on Pt(111), designated Ni–Pt–Pt(111), was found to show superior reforming activity in comparison to Pt(111), subsurface monolayer Pt–Ni–Pt(111), and a Ni(111) film [2]. The stability of these monolayer surfaces was also studied by Menning et al. in which it was found that the Ni monolayer in the Pt–Ni–Pt(111) surface segregates to the surface and forms the Ni–Pt–Pt(111) surface under oxidizing conditions [3]. Another unique property of Pt-Ni bimetallic systems is that the Pt–Ni–Pt(111) surface binds hydrogen more weakly than either parent metal allowing for novel low-temperature hydrogenation pathways [4]. The objective of the current study is to extend previous investigations on single crystal surfaces to supported catalysts in an attempt to understand structure-property relationships and to bridge the materials gap.
For this study, Pt, Ni, and Pt-Ni bimetallic catalysts supported on g-Al2O3 were synthesized via incipient wetness impregnation using both sequential impregnation as well as co-impregnation for the bimetallic catalysts. Previous work by Shu et al. reported on the effect of impregnation sequence for Pt-Ni bimetallic catalysts, and it was found that for Pt/Ni atomic ratios of 1/1 resulted in the formation of bimetallic catalysts using both sequences. However for Pt/Ni atomic ratios of 3/1, bimetallic catalysts were not formed for the Ni first sequence [5]. Therefore in this work the Pt/Ni atomic ratios were chosen to be 1/3 and 1/10 in order to determine the effect of metal loading on catalytic activity and bimetallic formation.
In the work presented here catalytic activity was characterized both through gas phase batch and flow reactor studies. In an attempt to observe the unique activities displayed by the Pt-Ni bimetallic systems in surface science experiments, both hydrogenation and oxygenate reforming reactions were used as probe reactions to characterize the catalysts. In-situ extended x-ray absorption fine structure (EXAFS) measurements were used to determine the extent of bimetallic formation and to relate the catalytic activity to the catalyst structure. Additional characterization included CO chemisorption to quantify catalytic surface area and transmission electron microscopy (TEM) to obtain average particle sizes.
[1] G. W. Huber, J. W. Shabaker, S. T. Evans, J. A. Dumesic, Aplied Catalysis B: Environmental, 62 (2006) 226.
[2] O. Skoplyak, M. A. Barteau, J. G. Chen, Journal of Physical Chemistry B, 110 (2006) 1686.
[3] C. A. Menning, J. G. Chen, Journal of Chemical Physics, 128 (2008) 164703.
[4] H. H. Hwu, J. Eng Jr., J. G. Chen, Journal of American Chemical Society, 124 (2002) 702.
[5] Y. Shu, L. E. Murillo, J. P. Bosco, W. Huang, A. I. Frenkel, J. G. Chen, Applied Catalysis A: General, 339 (2008) 169.